A proprietary encryption algorithm known as RC4.
RC4 is a symmetric stream cipher: the same algorithm shall be used for both encryption and decryption, and the algorithm does not change the length of the data.

AES

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The AES (Advanced Encryption Standard) algorithm (beginning with PDF 1.6). AES is a symmetric block cipher: the same algorithm shall be used for both encryption and decryption, and the length of the data when encrypted is rounded up to a multiple of the block size, which is fixed to always be 16 bytes, as specified in FIPS 197, Advanced Encryption Standard (AES).

Strings and streams encrypted with AES shall use a padding scheme that is described in Internet RFC 2898, PKCS #5: Password-Based Cryptography Specification Version 2.0. For an original message length of M, the pad shall consist of 16 - (M mod 16) bytes whose value shall also be 16 - (M mod 16).

The Cipher Block Chaining (CBC) mode, which requires an initialization vector, is used. The block size parameter is set to 16 bytes, and the initialization vector is a 16-byte random number that is stored as the first 16 bytes of the encrypted stream or string.